Most people are vaguely aware
that shutter speeds aren't always as fast as they are marked,
that ISO speeds can vary, and that overall, photography isn't
that precise a science. What they often don't know is just how
far-out things can be.

Black and white film speeds are the best place to start, as they exhibit
the greatest variations. ISO speeds are determined for a specific shadow
density (0.10 above film base plus fog) and a specific contrast (gamma
as near 0.62 as makes no difference). Normally, they are pretty accurate,
but a lot depends on the developer. For example, my favorite film is
Ilford HP5 Plus. It is nominally ISO 400. In a middle-of-the-road developer
this is a pretty fair figure.

Kiniszi
Castle, Hungary. When you are shooting on 6x9cm, as Frances
Schultz was here, you don't need to worry too much
about exposure as long as it is generous. Grain will be
up and sharpness will be down if you overexpose, but with
rather less than a 4x enlargement to give an 8x10"
print, who cares? (Alpa 12 S/WA, 58mm f/5.6 Schneider
Super Angulon XL, Ilford HP5 Plus printed on Ilford Multigrade
Warmtone, sepia toned with sulfide.)

But speed increasing developers
such as Ilford's own Microphen or DD-X or Paterson's excellent
FX-50 can give ISO 650, still meeting ISO criteria for shadow detail and
contrast, and a fine-grained developer can easily drop the speed to ISO
200. Some manufacturers use speed increasing developers for ISO tests--Fomapan's
excellent ISO 200 film is 160 or even 125 in Kodak D-76--but (unsurprisingly)
none use speed reducing developers.

That's not all. In the days when emulsion technology was less precise
than it is today, emulsion speeds could and did vary by +/- 1/3 stop from
batch to batch, so ISO standards permit a film to be sold at a nominal
speed up to 1/3 stop different from its measured ISO speed. Today, such
variations are unusual, and with black and white they are normally skewed
to reduce the risk of underexposure: you might find that Ilford HP5 Plus
was nearer ISO 500 than ISO 400, but you'd be very unlikely to find
a batch slower than ISO 400.

Take these factors together, and the very same roll of HP5 could legitimately
be sold at anything from ISO 160 (Perceptol, less 1/3 stop poetic license)
to ISO 800 (Microphen, plus 1/3 stop poetic license). And this assumes
you want ISO contrast, which you may not.

If you use a diffuser enlarger, for example, you may prefer to develop
your film to a contrast (gamma) of 0.70, which gives maybe another 1/3
stop in useful speed. This is no longer an ISO speed because the contrast
no longer meets the ISO criterion.

Developing Procedure
Variables
So far, though, we haven't looked at possible variations in time,
temperature, and agitation. Most clocks are accurate to +/- 2 percent
at worst, so this can safely be ignored: but when do you start and stop
timing? From just before you tip the developer in? Or just after? Or after
the first few seconds of agitation, and banging the tank on the table
to dislodge air bubbles? When do you drain? Do you start 15 seconds before
the processing time is up? Or do you start when it is up? Your "seven
minutes" could easily differ from my "seven minutes"
by half a minute.

How accurate is your thermometer? Even the best thermometers can disagree
by 0.2ÞC, and +/- 1ÞC is nothing unusual, so two photographers
might measure the very same developer at 68Þ and 70Þ.

What about agitation? Constant agitation is reckoned to demand 10-15 percent
less development time than agitating for 10 seconds each minute. Use the
same development times for both, and the film that is constantly agitated
will receive significantly more development.

Do you use pre-washes or presoaks? These are very unpredictable. They
normally entail about 15 percent longer in the developer. Development
accelerators, as found in most ultra-fast films such as Kodak TMZ P3200
and Ilford Delta 3200, may be washed out to some extent: even with a considerable
increase in development time, you may still see a loss of effective speed.

Restaurant
Cornee. Ilford XP2 Super is incredibly forgiving. Most
novices at black and white would do far better to start
out by learning to take good pictures on XP2 than to start
looking for a degree of precision in development and exposure
that doesn't exist. As far as I recall I used a
Leica M-series (this was before modern Voigtländers
came out) and a 21mm f/4.5 Zeiss Biogon for this shot
in Bruges/Brugge, Belgium.

Printing Variables
There is no room here to discuss fully what happens at the printing stage.
Do you use a condenser enlarger, condenser-diffuser, or pure diffuser?
How contrasty is your lens? How much flare is there in your enlarger/lens
system? Which paper do you use? One manufacturer's Grade 2 may deliver
the same contrast as another's Grade 3. And your developer choice
can add half a grade of contrast to this, or wipe off a grade or more.
For that matter, top-quality prime lenses for rangefinder cameras, such
as Leica and Voigtländer, are much contrastier than most reflex lenses,
especially zooms: about one paper grade contrastier.

Color Variables
So much for black and white. In color, the likely variations are smaller,
but processing variations can and do account for speed changes as large
as +/- 1/3 stop, or an overall spread of 2/3 stop. Even +/- 1/2 stop is
quite possible, still with perfectly believable colors, if you do your
own processing.

Shutter Speed
Now let's look at shutter speeds. If a marked 1/2000 sec on a new
camera is a true 1/2000, it's doing very well. As often as not,
it's 1/1600 (1/3 stop slow), and after a few months or years it
is quite likely to drop to 1/1200 (2/3 stop slow) and stay there. Likewise,
1/1000 is often 1/800 (1/3 stop slow) and on a well-worn camera it can
be 1/650 (2/3 stop slow). At 1/500 and 1/250 you may again find 1/3 stop
slow, though at 1/125 and longer they are normally spot on, unless the
camera is very "tired."

Meter Calibration
Next, metering. Once, I tested three Weston Master meters, and they were
all within 1/6 stop of one another, but this is unusual. Normally, I'd
regard differences of 1/3 stop as good, 2/3 stop as tolerable, and a full
stop as nothing unusual.

This is before you look at metering technique. Give two people the same
meter (handheld or in camera) and their readings are more than likely
to differ by 1/3 stop. With some meters and metering techniques, the difference
can be as much as a stop. People who don't know how to use spot
meters, and therefore try to meter mid tones, will often disagree by two
stops or more.

The Sum Of The Parts
Now let's add all this together. Let's assume that you are
shooting Ilford HP5 Plus and developing it in Microphen: that's
a 2/3 stop gain, even before you decide to develop it to a slightly higher
contrast to suit your diffuser enlarger and gain another 1/3 stop. This
adds up to a full stop in extra speed.

Now let's assume that the shutter is 1/3 stop slow, which is the
same as another 1/3 stop speed gain in terms of the exposure on the film.
Add in a comparatively minor 1/3 stop variation in meter accuracy, and
another 1/3 stop variation as a result of your personal metering technique.
This is another complete stop. In other words, you can rate the film at
EI 1600, and still get the best possible exposure for your purposes.

Or let's assume that you prize fine grain, and use a condenser enlarger.
You still go for Ilford HP5 Plus, for its unparalleled tonality, but in
a fine-grained developer that gives ISO 200: one stop lost. Your shutter
is spot on (they seldom run fast) but your meter is 1/2 stop out and you
always meter conservatively, rounding your exposures to give a little
more than the meter indicates: this also knocks 1/2 stop off, for yet
another whole stop. You need to rate the film at EI 100 in order to get
the results you want: four full stops slower than the speed described
in the other example.

How It All Cancels
Out
The real puzzle in all this is that so many people get exposures that
are so good. How do they do it? There are three answers.

The first is that often, the errors cancel out: they are seldom all in
the same direction. The film is a tiny bit "hot," but you
always underdevelop a bit anyway; your meter indicates 1/3 stop too little
exposure, but your shutter gives 1/3 stop too much.

The second, is that photographers make the necessary adjustments, almost
without thinking. They know that one camera-lens-film combination needs
to be rounded up from the meter's recommendation, while another
needs to be rounded down. They think of it as an exception, not a general
rule, but they do it anyway. This is why your meter can differ a stop
from mine, and we still both get good exposures.

The third, and quite possibly the most important, is that photography
really isn't all that precise: the latitude in the system (especially
with pos/neg processes) masks the inaccuracies. In other words, photography
is more like cookery than science. But very few photographers seem to
want to believe this...